Retirement Plans - Components of Net Pension and Other Postretirement Benefit Plan (OPEB) Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension expense (income)
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.1	$ 0.1	$ 0.5	$ 0.5
Interest cost	7.7	8.0	23.4	23.7
Expected return on plan assets	(12.4)	(12.7)	(37.6)	(37.6)
Amortization, net	2.0	1.9	5.9	5.4
Settlements	(0.2)		(1.5)
Net pension income	(2.4)	(2.7)	(6.3)	(8.0)
OPEB expense (income)
Defined Benefit Plan Disclosure [Line Items]
Service cost	(0.3)	0.4	0.5	1.0
Interest cost	2.5	2.7	7.7	8.3
Expected return on plan assets	(3.5)	(3.4)	(10.5)	(10.1)
Amortization, net	(0.8)	(0.7)	(2.2)	(2.2)
Other	(5.1)		(5.1)
Net pension income	$ (7.2)	$ (1.0)	$ (9.6)	$ (3.0)